1. Field of the Invention
The invention relates generally to a mass flow measurement and control, and more particularly, to a mass flow measurement and control device based on the Coriolis force effect and having an integrated flow control valve with associated sense, control and communication electronics.
2. Description of Related Art
Mass flow measurement based on the Coriolis force effect is achieved in the following manner. The Coriolis force results in the effect of a mass moving in an established direction and then being forced to change direction with a vector component normal to the established direction of flow. This can be expressed by the following equation:
{right arrow over (F)}c={right arrow over (M)}xc3x97{right arrow over (xcfx89)}
Where {right arrow over (F)}c (the Coriolis force vector) is the result of the cross product of {right arrow over (M)} (the mass flow vector) and {right arrow over (xcfx89)} (the angular velocity vector).
In a rotating system, the angular velocity vector is aligned along the axis of rotation. Using the xe2x80x9cRight Hand Rulexe2x80x9d, the fingers define the direction of rotation and the thumb, extended, defines the angular velocity vector direction. In the case of the typical Coriolis force flow sensor, a tube, through which fluid flow is to be established, is vibrated. Often the tube is in the shape of one or more loops. The loop shape is such that the mass flow vector is directed in opposite directions at different parts of the loop. The tube loops may, for example, be xe2x80x9cUxe2x80x9d shaped, rectangular, triangular or xe2x80x9cdeltaxe2x80x9d shaped or coiled. In the special case of a straight tube, there are two simultaneous angular velocity vectors that are coincident to the anchor points of the tube while the mass flow vector is in a single direction.
The angular velocity vector changes directions since, in a vibrating system, the direction of rotation changes. The result is that, at any given time, the Coriolis force is acting in opposite directions where the mass flow vectors or the angular velocity vectors are directed in opposite directions. Since the angular velocity vector is constantly changing due to the vibrating system, the Coriolis force is also constantly changing. The result is a dynamic twisting motion being imposed on top of the oscillating motion of the tube. The magnitude of twist is proportional to the mass flow for a given angular velocity.
Mass flow measurement is achieved by measuring the twist in the sensor tube due to the Coriolis force generated by a fluid moving through the sensor tube. Known devices use magnet and coil pairs located on the flow tube where the Coriolis force""s induced displacement is expected to be greatest. The coil and magnet are mounted on opposing structures, for example, the magnet is mounted on the tube and the coil is mounted on the stationary package wall. The magnet will move in and out of the coil, inducing a current in the coil. This current is proportional to the velocity of the magnet relative to the coil. Since this is a velocity measurement, the velocity, and thus the signal, is at the maximum when the flow tube crosses its rest point (zero crossing). The Coriolis force induced twist causes a phase shift in the velocity signal that is detected by measuring the difference in the zero crossing times between the two velocity sensors. In practice this places a large accuracy burden on the time measurement circuitry. This may limit the ultimate sensitivity of mass flow measurement by this technique.
Further, the flow rate capabilities of known devices based on Coriolis technology often are limited to flow rates that are higher than desired for many applications. Moreover, existing Coriolis mass flow measuring devices only provide for mass flow sensing with no integral flow control capabilities. It has been left to the user to provide any means for controlling flow.
The present invention addresses shortcomings associated with the prior art.
In one aspect of the present invention, a Coriolis mass flow sensor includes a flow sensor tube, a drive device situated relative to the flow sensor tube so as to cause the flow sensor tube to vibrate, and capacitance displacement gauges situated relative to the flow sensor tube so as to measure the twist in the flow sensor tube due to Coriolis force. In specific embodiments, electromagnetic, electrostatic, acoustic, and/or piezoelectric drives are used to vibrate the flow sensor tube. In still further embodiments, piezoelectric devices are used both to vibrate the flow sensor tube and measure the twist in the flow sensor tube.
In another aspect of the present invention, a Coriolis mass flow controller includes a flow sensor tube, a drive device situated relative to the flow sensor tube so as to cause the flow sensor tube to vibrate and a position sensing device situated relative to the flow sensor tube so as to measure the twist in the flow sensor tube due to Coriolis force. The Coriolis mass flow controller further includes an integrated flow control device adapted to receive fluid from the flow sensor tube. Such an integrated flow valve with associated sensing electronics provide a more compact package that is easier to use and provides significant improvement in dynamic performance.